The present invention relates to a mask holder for especially large-surface substrates, especially for the microstructuring of organic electroluminescent materials (OLED) on preferably large-surface substrates, especially screens, displays and the like by means of vacuum-coating processes.
In the production of so-called OLED displays or screens, which utilize the light-emitting properties of organic, electroluminescent materials (OLED), it is necessary to apply organic, electroluminescent materials with a corresponding micro-structure on a transparent substrate, such as glass, such that the electroluminescent structures may be actuated by corresponding electrode layers and stimulated to emit light.
The micro-structured, organic, electroluminescent materials are usually applied by means of vacuum-coating processes, with the microstructures usually generated by corresponding shadow masks. These masks need to be placed on the substrates in a suitable manner, to be transported along with them through the corresponding coating chambers, and then removed again from them in order that, following possible cleaning, they may be reused.
On account of the small dimensions of the micro-structuring, very accurate positioning of the masks is essential, especially when so-called RGB displays are manufactured in which pixel areas for red, green and blue light have to be generated in close proximity to each other for chromatic presentation on the display. In addition, the vacuum-coating processes require a high degree of purity, such that it must be ensured that the handling devices for the masks do not generate any unnecessary particles.
A magnetic holding device for foil masks is described in DE 297 07 686 U 1. In the described holding device a foil mask is arranged on a substrate carrier by means of positioning pins on which said mask, in turn, the substrate to be coated lies. Above the substrate is an arrangement of magnets that serves to press the full surface of the ferromagnetic mask against the substrate, which is provided between mask and magnet arrangement. In such an arrangement for a mask holder, the mechanical attachment of the mask to the substrate carrier promotes the generation of undesirable particles and, moreover, such an arrangement is unfavourable for simple, quick and precise arrangement and removal of the mask from the substrate.
Further, EP 1 202 329 A 2 refers to a mask arrangement in which a mask is pressed across its full surface against the substrate by means of an arrangement of magnets behind the substrate carrier. The masks described therein may feature a peripheral frame by means of which the mask is laterally clamped in order to impart a certain degree of stability to the thin foil from which the mask is usually made. Although such magnetic attraction of the mask against the substrate enables full contact to be obtained between the mask and the substrate and hence freedom from blisters, and sharp coating edges, such an arrangement is not optimal for rapid, precise handling with regard to mask changing, especially under vacuum conditions. Moreover, the magnets merely serve the purpose of creating full-surface, blister-free contact between the mask and the substrate, but not that of holding in a dynamic system in which the substrate on the substrate carrier is moved with the mask through the coating area.